The present disclosure relates to a developing device for developing an electrostatic latent image by using toner, an image forming apparatus including the developing device, and a method for controlling the developing device.
There is an image forming apparatus such as a multifunction peripheral, a copier, a printer, or a facsimile, which forms an electrostatic latent image on a photosensitive drum and develops the electrostatic latent image by toner so as to perform printing. Further, the image forming apparatus which uses toner for printing is equipped with a developing roller that carries toner. Further, an AC voltage is applied to the developing roller. Thus, the toner is scattered from the developing roller toward the photosensitive drum. Then, the electrostatic latent image is developed. Further, a duty ratio of the AC voltage applied to the developing roller may be changed in the case of necessity.
There is known the following image forming apparatus, which changes the duty ratio of the AC voltage applied to the developing roller. Specifically, there is known an image forming apparatus including an image carrying member, a charging member for charging the image carrying member, a developing means including a developer carrying member for carrying developer, so as to develop an electrostatic latent image farmed on the image carrying member by the developer, and a bias applying means for applying a bias to the developer carrying member. When the electrostatic latent image is developed, the bias applying means applies an AC bias so that the developing means can collect the developer from the image carrying member. As to a duty ratio of the AC bias applied to the developer carrying member by the bias applying means when the developer is collected, a ratio of potential for moving the developer from the developer carrying member to the image carrying member is larger than that when the electrostatic latest image is developed.
In order to scatter the toner, a periodically changing AC voltage is applied to the developing roller. A peak-to-peak voltage of the AC voltage may be approximately 1 to 2 kV. Further, in order to generate the AC voltage having a peak-to-peak voltage of one to a few kilovolts, a transformer is used in many cases. First, a control signal (for example, a clock signal) is generated, in which a high potential state and a low potential state are periodically repeated. Next, the control signal from which a DC component is removed by a capacitor is supplied to a primary side of the transformer. Then, a stepped-up AC voltage from a secondary aide of the transformer is applied to the developing roller.
Here, in view of preventing a leakage between the photosensitive drum and the developing roller (preventing a discharge) or preventing unevenness of a toner image, there is a case where a duty ratio of the AC voltage applied to the developing roller should be changed in accordance with the state. In this case, when a duty ratio of the control signal is changed, the duty ratio of the AC voltage applied to the developing roller is changed.
However, it is known that when the duty ratio of the control signal is changed, a large current is apt to flow in a circuit or an element such as the transformer or the capacitor of the developing device. In particular, when a larger transient change of the duty ratio occurs, a large current exceeding a rated current flows more easily in the developing device. Further, when a large current flows, the circuit in the developing device may be broken down. It is considered that one of the causes of such large current due to the change of the duty ratio is bias magnetism or magnetic saturation of the transformer, which is generated when an imbalanced voltage (with biased energy) is applied to the primary side of the transformer. When the bias magnetism is generated so that a magnetic flux is biased, there is a state as if the transformer is biased by a DC voltage. Further, when the magnetic saturation is generated by the bias magnetism in the transformer, the inductance of the transformer becomes very small. Therefore, it is necessary to prevent the magnetic saturation from being generated in the transformer so that a large current does not flow in the developing device.
Note that the above-mentioned known image forming apparatus changes the duty ratio. However, there is no consideration about the magnetic saturation generated when the duty ratio is changed. Therefore, the problem caused by changing the duty ratio of the control signal cannot be solved. In addition, because the above-mentioned known image forming apparatus applies a developing bias to the developer carrying member, it is necessary to prepare a plurality of types of power supplies. However, with this structure, it is necessary to prepare a plurality of systems (step-up circuits including the transformer and the like) for applying high voltages to a developing roller 21 for individual patterns of the developing bias applied to the developing roller 21. Therefore, the conventional image forming apparatus has a problem that a high voltage generating circuit becomes a large scale and that manufacturing cost is increased.